1. Field of the Invention
The present invention relates generally to a method of performing optical proximity correction for preparing a mask projected onto a wafer by photolithography, and more specifically to a method of performing optical proximity correction for preparing a mask projected onto a wafer by photolithography that prepares the mask overlapping a boundary of two structures in the wafer.
2. Description of the Prior Art
The minimum feature sizes of integrated circuits (ICs) have been shrinking for years. Along with this size reduction, various process limitations have made IC fabrication more difficult. One area of the fabrication technology in which such limitations have emerged is photolithography. In the semiconductor fabrication process, lithography processes are important steps to transfer integrated circuit layouts to semiconductor wafers. Generally, a wafer manufacturing company designs a mask layout according to an integrated circuit layout; and then fabricates a mask having the designed mask layout. Afterwards, by way of lithography processes, the pattern on the mask (i.e. the mask pattern) is transferred to a photoresist layer on the surface of a semiconductor wafer with a specific scale.
More precisely, photolithography involves selectively exposing regions of a photoresist coated wafer to a radiation pattern, and then developing the exposed photoresist in order to selectively protect regions of wafer layers such as regions of a substrate, polysilicon or a dielectric.
A component of photolithographic apparatus is a mask which includes a pattern corresponding to features of one layer in an IC design. Such mask typically includes a transparent glass plate covered with a patterned light blocking material such as chromium. The mask is placed between a radiation source producing radiation of a pre-selected wavelength and a focusing lens. A photoresist covered wafer is placed beneath the focusing lens. When the radiation from the radiation source is directed onto the mask, light passes through the glass, i.e. regions having no chromium patterns, and is projected onto the photoresist covered wafer. In this manner, an image of the mask is transferred to the photoresist.
The photoresist is provided as a thin layer of radiation-sensitive material that is spin-coated over the entire wafer surface. The resist material is classified as either positive or negative depending on how it responds to light radiation. Positive photoresist becomes soluble and is thus more easily removed in a development process when exposed to radiation. Negative photoresist, in contrast, becomes less soluble when exposed to radiation. Consequently, a developed negative photoresist contains a pattern corresponding to the transparent regions of the mask.
As the complexity and the integration rate of the integrated circuits continue to progress, the size of every segment of a mask pattern is designed to be smaller. However, the exposure limit of every segment fabricated by exposure is limited to the resolution limit of the optical exposure tool used during the transfer step of the mask pattern. As light passes through the mask, it is refracted and scattered by the feature edges (chromium edges), thereby causing the projected image to show some roundings and other optical distortions. One problem that easily arises during the exposure of a mask pattern with high-density arranged segments to form a pattern on a photoresist is the optical proximity effect. Resolution losses occur because of overexposure or underexposure that induces deviations of the original pattern on the photoresist. Many saving methods have been used to improve the deviation caused by the optical proximity effect in order to improve the quality of the transferred pattern. The most popular method is the optical proximity correction (OPC). There has been a variety of commercial optical proximity correction softwares that can theoretically correct the mask pattern in order to obtain a more accurate pattern on a wafer. First, the digital pattern is evaluated with a software to identify regions where optical distortion will result. Then, the optical proximity correction is applied to compensate the distortion. The resulting pattern is ultimately transferred to the mask.